1. Field of the Invention
The present invention relates generally to read heads for magnetic data storage mediums, and more particularly to a read head using exchange coupling to stabilize the free magnetic layer of the read head.
2. Description of the Prior Art
A computer disk drive stores and retrieves data by positioning a magnetic read/write head over a rotating magnetic data storage disk. The head, or heads, which are typically arranged in stacks, read from or write data to concentric data tracks defined on surface of the disks which are also typically arranged in stacks. The heads are included in structures called “sliders” onto which the read/write sensors of the magnetic head are fabricated. The slider flies above the surface of the disks on a thin cushion of air, and the surface of the slider which faces the disks is called an Air Bearing Surface (ABS).
The goal in recent years is to increase the amount of data that can be stored on each hard disk. If data tracks can be made narrower, more tracks will fit on a disk surface, and more data can be stored on a given disk. The width of the tracks depends on the width of the read/write head used, and in recent years, track widths have decreased as the size of read/write heads has become progressively smaller. This decrease in track width has allowed for dramatic increases in the recording density and data storage of disks.
Recent read heads typically use a tunnel junction sensor, also known as a “tunnel valve”, abbreviated “TV”, for reading the magnetic field signals from the rotating magnetic data storage disk. The sensor typically includes a nonmagnetic tunneling barrier layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. The pinned layer in turn is fabricated on an antiferromagnetic (AFM) pinning layer which fixes the magnetic moment of the pinned layer at an angle of 90 degrees to the air bearing surface (ABS). The tunnel junction sensor is itself typically sandwiched between ferromagnetic first and second shield layers. These first and second shield layers also serve as first and second electrical lead layers, and are connected to the tunnel junction sensor for conducting a tunneling current through it. The tunneling current is preferably configured to conduct Current Perpendicular to the Planes (CPP) of the film layers of the sensor, as opposed to a sensor where a sense Current In the Planes (CIP) or parallel to film layers of the spin valve sensor. The CPP configuration is attracting more attention lately, as it can be made to be more sensitive than the CIP configuration, and thus is more useful in higher densities of tracks and data.
The magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic field signals from the rotating magnetic disk. The sensitivity of the tunnel junction sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the tunnel junction sensor from minimum resistance to maximum resistance and R is the resistance of the tunnel junction sensor at minimum resistance.
The free layer material is very soft material, magnetically speaking, with very low coercivity, which is a measure of the minimum field strength necessary to make changes in the orientation of the magnetic domains. The free layer material necessarily must have this quality, as it is this layer's changes in magnetic alignment in response to the magnetic domains in the data disk that leads to changes in resistance, which is how the data is read. This material may therefore very easily fall into a multidomain state, where alignment of magnetic domains is not uniform, or in other words, to become demagnetized. It is desirable however that the magnetization of the free layer be biased to be uniform throughout the entire layer. When the tunneling current is conducted through the sensor without magnetic field signals from the rotating magnetic disk, this is known as the quiescent position of the magnetic moment of the free layer. The preferred biased direction is parallel to the ABS.
If the free layer material is allowed to assume a multidomain state, the read signal will be degraded. In order to maintain the biased alignment during the quiescent state to produce better signal quality, there must be some external field applied to the free layer. The application of this external field is known as “stabilization”.
It is possible to stabilize the free layer by applying a layer of anti-ferromagnetic (AFM) material, but this method can have disadvantages that result from the thickness of the AFM material, which is typically large, and the strength of the biasing field produced, which must be large enough to stabilize the free layer material without pinning it too strongly. There may also be improvements in performance if the net magnetic moment of the layer providing exchange coupling to the free layer is kept near zero, as the magnetic system will be more stable. Once the magnetic moment is near zero, the material does not have shape demagnetizing and therefore retains pinning and stability to very narrow track widths and also does not demagnetize at elevated temperatures.
Thus there is a need for a stabilizing layer or layers which produces a proper biasing field, which is thin and does not contribute greatly to the overall head thickness and which preferably has a net magnetic moment very near zero.